Why is Boeing 737 Max 8 Grounded Worldwide?

The short answer: no one can be sure that the planes are safe.

Forty-seven airlines around the world have at least one Boeing 737 Max 8 airplane in their fleet, and all 47, complying with directives from national aviation authorities, have grounded those planes after two crashes in five months.

Lion Air Flight 610 went down in the Java Sea in October, just 12 minutes after takeoff. On Sunday, Ethiopian Airlines Flight 302 also crashed shortly after takeoff from Addis Ababa Bole International Airport. There were no survivors on either flight, and a total of 346 people lost their lives.

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Per aviation publication The Air Current, the issues with the 737 Max 8 stem from competition with Airbus. In 2011, Boeing had plans to design a brand-new plane to replace the 737, but after American Airlines decided to add Airbus planes to its previously Boeing-only fleet, the company was under a lot of pressure to deliver a more fuel-efficient single-aisle plane quickly, particularly given its existing major investment in the development of larger planes like the 787 Dreamliner.

So instead of designing a new plane, Boeing decided to swap out the engines on the 737 for more fuel efficient models. The new engines necessitated a chain reaction of engineering adjustments, but test flights revealed that the heavier engines altered the aerodynamics of the plane and nudged the nose higher, leading to an increased risk of stalling under certain conditions.

The company’s solution: the Manuevering Characteristics Augmentation System, an automated safety feature. As designed, an onboard sensor is automatically activates horizontal stabilizers when the plane is at an angle that makes a stall likely. The stabilizers point the nose of the plane down, reducing such a risk.

A preliminary report on the Lion Air flight shows that a faulty sensor triggered the MCAS when the plane was not pitched too high, effectively causing the crash by forcibly pointing the nose of the plane down toward the ground.

No such report is yet available for the Ethiopian Airlines crash, though the New York Times reports that the crashes happened under similar circumstances.

Boeing is working on a software update for the 737 Max that includes changes to the MCAS. If it’s enough to satisfy aviation officials, the planes will return in the air, but no one know if it will be enough or when such a decision might happen. Until then, 737 Max 8 planes around the world will remain grounded.